Semiconductor chips or dies typically are encapsulated in a package that protects the chips from the surrounding environment. The packages typically include leads or other connection points that allow the encapsulated die to be electrically coupled to another electronic component, e.g., a printed circuit board. Typically, the leads extend laterally outwardly in a flat array that is part of a lead frame. Leaded packages include a semiconductor die, which may be attached to the lead frame either by seating the die on a die paddle or attaching the die directly to the leads, e.g., via a die attach adhesive in a leads-over-chip attachment. Some or all of the terminals of the semiconductor die then may be electrically connected to leads of the lead frame, e.g., by wire bonding. The connected lead frame and die may then be encapsulated in a mold compound to complete the packaged microelectronic component assembly. In most common applications, the leads extend outwardly from the mold compound, allowing the features of the semiconductor die to be electrically accessed. In most applications, the lead frame finally will be trimmed and formed into a desired configuration.
FIGS. 1-4 schematically illustrate one microelectronic component assembly design that has been on sale for more than one year. In the drawings, FIG. 1 is a schematic top elevation view, FIG. 2 is a schematic perspective view, and FIGS. 3 and 4 are schematic cross-sectional views taken along lines 3-3 and 44 of FIG. 1, respectively.
The microelectronic component assembly 10 shown in FIGS. 1-4 includes a microelectronic component 20, a mold compound 30, and a lead frame 50. The microelectronic component 20 may be any of a wide variety of known devices. In the illustrated embodiment, the microelectronic component 20 is typified as a semiconductor die having a plurality of terminals 24 arranged on an active surface 22 thereof. These terminals are arranged in a generally longitudinally disposed array to facilitate electrical connection of the terminals 24 to the leads 60 of the lead frame 50.
The lead frame 50 includes a pair of opposed end members 52a and 52b, a first set of leads 60a, and a second set of leads 60b (the first and second sets of leads being collectively referred to as leads 60). Each of the end members 52 includes a body 54 having an inner edge 56.
Each of the leads 60 includes an inner length 62 and an exposed length 64. The exposed length 64 of each lead 60 includes a tip portion 66 adjacent to its outer edge. These tip portions 66 may be connected to one another by a lead tip bar 80a or 80b, which extends between and connects the tip portions 66 of adjacent leads 60 to one another.
The inner ends of the lead inner lengths 62 may be attached to the microelectronic component 20 by a die attached adhesive 28 or the like. Selected terminals 24 of the microelectronic component 20 may be electrically coupled to selected leads 60 in any suitable fashion, e.g., using a plurality of bonding wires 26.
A first dam bar 70a may extend between and connect the exposed lengths 64 of the first set of leads 60a and a second dam bar 70b may extend between and connect the exposed lengths 64 of the second set of leads 60b. The inner edges 56 of the end members 52a, b and the dam bars 70a, b define a molding perimeter. As is known in the art, such a molding perimeter is designed to interface with a mold used to form and shape the mold compound 30, e.g., by transfer molding techniques. The mold compound 30 has a peripheral edge 32 that includes first and second longitudinal sides 34a and 34b and first and second transverse sides 34c and 34d. This peripheral edge 32 is typically spaced slightly inwardly from the molding perimeter, with the longitudinal sides 34a and 34b spaced slightly inwardly from and extending generally parallel to the adjacent dam bar 70a or 70b, respectively. The first transverse side 34c may be positioned adjacent the inner edge 56 of the first end member 52a, and the second transverse side 34d is positioned adjacent the inner edge 56 of the second end member 52b. 
The microelectronic component 20, leads 60, and mold compound 30 together define a package 15. FIGS. 1-4 illustrate a single microelectronic package 15 associated with the lead frame 50. As is well known in the art, a plurality of packages 15 may be arranged in an array on a single lead frame. The lead frame 50 shown in FIGS. 1-4 is well suited for a linear array consisting of a single row of packages 15. It should be recognized, though, that lead frame members other than the end members 52 may be employed in a rectangular array with multiple rows and multiple columns of microelectronic packages 15.
FIGS. 1-4 illustrate a microelectronic component assembly 10 in which the lead frame 50 includes two sets of leads 60 extending laterally outwardly beyond opposite longitudinal sides 34a and 34b of the mold compound 30. Other microelectronic component assembly designs known in the art include leads that extend outwardly from each of the four sides of the mold compound encapsulating the microelectronic component. For example, U.S. Pat. No. 5,793,100 (the teachings of which are incorporated herein by reference) suggests several systems that employ lead frames having leads that extend outwardly from each of four rectilinear sides of a mold compound.
As is known in the art, the lead frame 50 typically is formed of a relatively thin (e.g., 0.10-0.5 mm) metal foil or the like. To improve structural integrity during manufacturing operation, the lead frame 50 of FIGS. 1-4 incorporates a plurality of relief straps 90 extending between the opposed end members 52. In particular, one of the relief straps 90 is disposed between each of the microelectronic components in an array of microelectronic components associated with the lead frame 50. In the illustrated embodiment, a first relief strap 90a extends generally parallel to the first dam bar 70a and is connected at one end to the first end member 52a and the other end to the second end member 52b. Similarly, a second relief strap 90b extends generally parallel to the second dam bar 70b and has one end connected to the first end member 52a and another end attached to the second end member 52b. Each of these relief straps 90 includes a flexible element 92 disposed approximately midway along its length. These flexible elements 92 may comprise a thin, Z-shaped length of the relief strap 90. When the relief strap 90 is subjected to stresses, as outlined below, such a flexible element 92 provides a preferred bending location for the relief strap 90, allowing it to flex more readily. In the specific design of FIGS. 1-4, the lead frame is formed of a metal foil having a thickness of about 0.127 mm (5 mils), the main body of each of the relief straps 90 has a width of about 0.90 mm, and each relief strap 90 has a width of only about 0.20 mm along the length of the flexible element 92.
The mold compound 30, lead frame 50, and the microelectronic component 20, typically have different coefficients of thermal expansion (CTEs). When the mold compound 30 is molded about the inner lengths 62 of the leads 60 and the microelectronic component 20 to cover or substantially encapsulate them, the mold compound 30 typically is introduced as a relatively hot molten plastic resin. As this resin cools, the differences in CTE between the mold compound 30, the microelectronic component 20, and the lead frame 50 places the lead frame 50 under thermally induced stress. As suggested by the dashed outline in FIG. 3, this stress can cause the packaged microelectronic component assembly 10 to bow. Although the extent of the bowing is not drawn to scale in FIG. 3, this bowing can have significant adverse consequences on the final microelectronic component assembly. The height B of the bow, i.e., the maximum deviation of the microelectronic component assembly 10 from an idealized flat configuration, can vary depending on a number of factors, including the material and dimensions of the lead frame 50, the material and dimensions of the microelectronic component 20, and the material and dimensions of the mold compound 30. In one specific implementation, a bow height B on the order of about 0.088 mm (3.45 mils) is not uncommon. When viewed in light of the 0.127 mm thickness of the lead frame, this represents a meaningful deviation of the microelectronic component assembly 10 from its idealized flat state.